1. Field of the Invention
The present invention relates to a microfocus X-ray generating device that is used for non-destructive X-ray imaging in the industrial field and for diagnostic applications in the medical field.
2. Description of the Related Art
It is known that X-rays are used for non-destructive testing in the industrial field and for radiography in the medical field because the internal structure of an object can be observed by utilizing the high penetrability of X-rays.
The resolution of an X-ray radiography depends, among other things, on the size of the radiation source of X-rays. Therefore, in order to observe a very small internal structure, a microfocus X-ray generating device that has a very small radiation source needs to be used.
In order to increase the brightness of an X-ray radiograph, the amount of X-ray radiation needs to be increased.
Traditionally, the amount of X-ray radiation has been increased by increasing the current of an electron beam that is made incident on a target.
Japanese Patent Laid-Open No. 8-96986 describes an X-ray generating device in which the amount of X-ray radiation is increased by using a multilayer target. In Japanese Patent Laid-Open No. 8-96986, a target (as illustrated in FIGS. 9A and 9B) is made from a silicon wafer or the like so as to form a thin-film portion. The thin-film portion is made thinner than other portions of the target so that an electron beam can pass therethrough. A multilayer target is formed by stacking the targets by using the thicker portions as spacers. An electron beam is made incident on the thin-film portion of each target of the multilayer target so as to generate multiple-interaction X-rays, whereby X-rays having high energy are generated.
However, in existing microfocus X-ray generating devices, when a high current electron beam is made incident on a very small focal spot, a target can be damaged and various adverse effects, such as a decrease in the degree of vacuum inside the device, are produced. Therefore, it is difficult to reduce the size of the radiation source while increasing the amount of X-ray radiation.
In the X-ray generating device described in Japanese Patent Laid-Open No. 8-96986, which uses a multilayer target, an electron beam is diffused when the electron beam passes through each of the targets included in the multilayer target. Therefore, the larger the number of targets in the multilayer target, the larger becomes the size of a focal spot formed on a target that is located on a side on which X-rays are output. As a result, it is difficult to reduce the size of the radiation source. Moreover, the diameter of an electron beam increases when the current of the electron beam increases because electrons repel each other owing to the charge thereof. Also in this respect, it is difficult to reduce the size of radiation source while increasing the amount of X-ray radiation.